Co-stimulation of IL-2 production by CD28 is independent of tyrosine-based signaling motifs in a murine T cell hybridoma.

In addition to the antigen-specific stimulus delivered by the TCR, T cells under most circumstances require a co-stimulatory signal for complete activation. CD28 can provide this signal, and the importance of CD28-mediated co-stimulation has been well documented both in vitro and in vivo, but the intracellular pathways downstream of CD28 are less well characterized. So far, maximal co-stimulation of IL-2 production has been attributed to tyrosine-based signaling motifs, either including the first cytoplasmic tyrosine residue that binds phosphatidylinositol 3'-kinase (PI3-K), or the third tyrosine residue. Here we describe results of the expression of murine CD28 receptor mutants in a CD28-deficient murine T cell hybridoma, A1.1. We show that in A1.1 cells co-stimulation of IL-2 production is independent of CD28 cytoplasmic tyrosine residues, since a mutant lacking all four cytoplasmic tyrosines is still able to induce a full co-stimulatory response. Using truncation mutants, this activity can be attributed to amino acids 183 to 194, a sequence containing a conserved diproline motif that may recruit SH3 domains of other signaling molecules like Grb2. Thus we have identified a novel pathway for CD28-mediated co-stimulation of IL-2 production that is independent of PI3-K activity and phosphotyrosine-based signaling motifs.